Milo: Use custom config caching layer

luci_config/server/cfgclient/backend/backend.go

Index: luci_config/server/cfgclient/backend/backend.go
diff --git a/luci_config/server/cfgclient/backend/backend.go b/luci_config/server/cfgclient/backend/backend.go
index c0e2557faf19c997b04a469026d26813b2241864..1417dba85ca0f997fcc6a4de44b474288f16cf82 100644
--- a/luci_config/server/cfgclient/backend/backend.go
+++ b/luci_config/server/cfgclient/backend/backend.go
@@ -7,6 +7,8 @@ package backend
 import (
 	"net/url"
 
+	"github.com/luci/luci-go/common/config"
+
 	"golang.org/x/net/context"
 )
 
@@ -63,6 +65,9 @@ type B interface {
 
 	// ConfigSetURL returns the URL for the specified config set.
 	ConfigSetURL(c context.Context, configSet string, p Params) (url.URL, error)
+
+	// GetConfigInterface returns the raw configuration interface of the backend.
+	GetConfigInterface(c context.Context, a Authority) config.Interface
 }
 
 // Factory is a function that generates a B given a Context.